The present invention generally relates to a method for forming aluminum bumps on a semiconductor structure and more particularly, relates to a method for forming aluminum bumps on a semiconductor structure by a chemical vapor deposition technique and a wet etch technique with significantly reduced number of processing steps.
In the fabrication of semiconductor devices, the ever increasing device density and the decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques for such devices. In recent years, a flip chip attachment method has been widely used in the packaging of semiconductor chips. In the flip chip attachment method, instead of attaching a semiconductor die to a lead frame in a package, an array of bumps is first formed on the surface of the die. The formation of the bumps may be carried out by a variety of methods depending on the electrically conductive material that is used to form the bumps. For instance, evaporation, electrodeposition, stencil printing, screen printing have all been used to form electrically conductive bumps on flip chips.
The more frequently utilized bump fabrication techniques are a metal deposition process and a plating process. To carry out either of the processes, a series of barrier and seed layers of metal are first deposited on the surface of the semiconductor wafer. These layers are later removed by a wet etching process everywhere except under the die pads and the layers are used to improve adhesion of subsequent layers and to form a barrier to stop metal diffusion from the bump material to the underlying die pad. In a typical bump forming process, a layer of a photoresist material is then deposited over the surface of the semiconductor wafer. A photo mask is then used to pattern the locations over each of the die pads that a bump is to be formed. An etching process, such as plasma etching is used to expose the die pads, while the openings in the photoresist layer determines the shape and height of the bump to be formed.
The electrically conductive bump, which is typically formed of gold or aluminum, can be electroplated or sputtered over the die pad and the barrier and seed layers. Once the plating or sputtering step is completed, a series of wet etching steps is used to remove the photoresist layer and the various barrier and seed layers that cover the remainder area of the wafer while the bumps protect the underlying material from being etched. While gold is the most commonly used material, other electrically conductive materials such as copper, tin-lead and aluminum as well as layered composites of these materials can also be utilized.
A conventional method for forming gold bump is illustrated in FIGS. 1Axcx9c1I. As shown in FIG. 1A, an input/output (I/O) pad 12 formed on a semiconductor substrate 14 is first provided for a semiconductor structure 10. On top of the I/O pad 12, is then deposited a passivation layer 16 of an insulating material. The passivation layer 16 is formed by a photolithographic method using a mask (not shown) to provide an opening 18 for the I/O pad 12. In the next step of the process, as shown in FIG. 1B, a diffusion barrier layer 20 of TiW is conformally deposited into the pad opening 18. On top of the TiW barrier layer 20, is then deposited a gold seed layer 22, as shown in FIG. 1C. Both the TiW barrier layer and the Au seed layer may be suitably deposited by using a sputtering technique or an electroplating technique. On top of the semiconductor structure 10, is then coated, most likely by a spin coating technique, a thick photoresist layer 24.
In the next step of the process, as shown in FIG. 1E, the photoresist layer 24 is patterned by a mask (not shown) and an opening 26 is formed by a dry etching method such as plasma etching. The opening 26 is then filled, by an electroplating process of Au, as shown in FIG. 1F. The photoresist layer 24 is then stripped by a dry etching method leaving the Au bump 28 exposed on the semiconductor 10. In the next two steps of the process, as shown in FIGS. 1H and 1I, the gold seed layer 22 is etched away by a wet etch method and then, the TiW barrier layer 20 is etched away by a wet etch method exposing only the gold bump 28 above the passivation layer 16.
The conventional gold bump forming process requires numerous photolithographic steps, numerous deposition steps and various dry etching and wet etching steps. It is a time consuming and laborious process which severely impacts the yield of the semiconductor device.
FIGS. 2Axcx9c2F demonstrates a conventional process for forming aluminum bumps which requires at least two photolithographic steps, a sputtering step and numerous etching steps. A conventional semiconductor structure 30 is first provided which has a metal pad layer, or an input/output layer 32 formed on a substrate 34. The I/O metal pad layers 32 may be advantageously formed of a conductive metal such as aluminum, or aluminum that has an alloy content, for instance, aluminum with less than 3 wt. % copper. On top of the I/O pad layer 32, is then deposited a passivation layer 36 with an insulating material such as SiO2, Si3N4, SiON, SOG or polyimide. The passivation layer 36 is preferably a photo-sensitive material such that it can be imaged by placing a mask 38 on top. This is shown in FIG. 2A.
After an opening 40 is formed by a dry etch method in the passivation layer 36, as shown in FIG. 2B, an aluminum layer 42 is sputter deposited into the opening 40 and on the top surface 44 of the remaining passivation layer 36. After the aluminum sputtering process is completed, a second photoresist layer 46 is deposited and patterned on the aluminum layer 42 for forming the aluminum bump. This is shown in FIG. 2D.
In the final steps of the conventional aluminum bump filing process, as shown in FIGS. 2E and 2F, a dry etch method, such as plasma etching is used to form the aluminum bump 50 by using the photoresist layer 46 to define the bump. The photoresist layer 46 is then subsequently removed in a wet etch process to complete the formation of the aluminum bump 50. The conventional aluminum bump forming process, as shown in FIGS. 2Axcx9c2F, therefore requires numerous photolithographic and other chemical processing steps which are time consuming and laborious leading to a low yield of the process.
It is therefore an object of the present invention to provide a method for forming aluminum bumps that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for forming aluminum bumps that does not require multiple steps of photolithography.
It is a further object of the present invention to provide a method for forming aluminum bumps that only requires a single photolithographic process.
It is another further object of the present invention to provide a method for forming aluminum bumps by a chemical vapor deposition process to selectively deposit aluminum.
It is still another object of the present invention to provide a method for forming aluminum bumps by a chemical vapor deposition process and a wet etch process.
It is yet another object of the present invention to provide a method for forming aluminum bumps by using a passivation layer and a photoresist layer with an opening therein as a mold for depositing aluminum and forming the bump.
It is still another further object of the present invention to provide a method for forming aluminum bumps that require a reduced number of processing steps than the conventional formation methods.
In accordance with the present invention, a method for forming aluminum bumps by chemical vapor deposition and wet etch is provided.
In a preferred embodiment, a method for forming aluminum bumps by chemical vapor deposition and wet etch can be carried out by the operating steps of first providing a pre-processed electronic substrate that has a plurality of input/output pads formed on top; depositing a passivation layer of an insulating material on top of the substrate; depositing a polyimide layer that has a thickness of at least 5 xcexcm on top of the passivation layer; imaging the polyimide layer and forming a plurality of openings in the passivation and the polyimide layer; depositing a metal that includes Al by a CVD technique into the plurality of openings; and removing the polyimide layer and forming a plurality of bumps that includes Al.
The method for forming aluminum bumps by chemical vapor deposition and wet etch may further include the step of forming the plurality of I/O pads in a metal that includes aluminum, or the step of depositing the passivation layer from a material selected from the group consisting of silicon oxide, silicon nitride and spin-on-glass, or the step of depositing the passivation layer in at least two sub-layers of Si3N4 and SiO2, or the step of depositing the passivation layer in two sub-layers of a first layer of Si3N4 in a thickness between about 5000 xc3x85 and about 8000 xc3x85 and a second layer of SiO2 in a thickness between about 2000 xc3x85 and about 4000 xc3x85. The method may further include the step of depositing the passivation layer to a thickness of at least 1 xcexcm, or the step of depositing the polyimide layer to a thickness of at least 5 xcexcm, or a thickness between about 5 xcexcm and about 10 xcexcm. The method may further include the step of forming the plurality of openings by plasma etch, or the step of depositing a metal including Al and Cu by CVD into the plurality of openings. The method may further include the step of removing the polyimide layer by wet etch, or by an etchant that includes HF and NH4F.
The present invention is further directed to a method for forming aluminum bumps by chemical vapor deposition and wet etch that can be carried out by the steps of providing a pre-processed semiconductor structure that has a plurality of I/O pads formed on top; depositing a passivation layer of an insulating material on top of the structure; printing a polyimide layer on top of the passivation layer exposing the portions of passivation layer that overlies the plurality of I/O pads; plasma etching through the passivation layer by using the polyimide layer as a mask forming a plurality of openings to expose the plurality of I/O pads; filling the plurality of openings with a metal that includes Al by a CVD technique; and removing the polyimide layer and exposing a plurality of Al bumps.
The method for forming aluminum bumps by chemical vapor deposition and wet etch may further include the step of forming the plurality of I/O pads with a metal that includes Al, or the step of depositing the passivation layer in a material that is selected from the group consisting of silicon oxide, silicon nitride and spin-on-glass. The method may further include the step of depositing the passivation layer in at least two sub-layers of Si3N4 and SiO2, or at least two sub-layers of a first layer of Si3N4 in a thickness between about 5000 xc3x85 and about 8000 xc3x85 and a second layer of SiO2 in a thickness between about 2000 xc3x85 and about 4000 xc3x85. The method may further include the step of printing the polyimide layer by screen printing or stencil printing, or the step of filling the plurality of openings with a metal that includes Al and Cu, or the step of removing the polyimide layer by a wet etch technique.